This investigation proposes to take a multi-systems approach to studying cold tolerance and thermal regulation in relation to age and sex differences. Male and female subjects between the ages of 20-70 years will be exposed on two occasions for 2 and 2.5 hours to environmental temperatures of 29, 20, 15 and 10 degrees centigrade while cardiovascular, respiratory, metabolic and endocrine responses are being monitored. Differences in physiological responses to cold will be evaluated on the basis of age, sex and body composition, and the different systems will be assessed in relationship to each other in order to obtain an integrated picture of age- and sex-related changes in cold tolerance. The proposed investigation will attempt to prescribe the lowest comfortable room temperature for people of different ages. It will provide valuable insight into the mechanisms responsible for age- and sex-related differences in cold tolerance, and the best possible means for ameliorating cold intolerance.